An infrared imaging device in which a light receiving part and signal processing part each formed on a different substrates connected by metal bumps, such as indium bumps, i.e., a hybrid infrared imaging device, has conventionally been utilized. However, in producing such a hybrid infrared imaging device, a troublesome process for connecting those substrates with the metal bumps is required. In addition, since the hybrid infrared imaging device is generally cooled down to about 77.infin. K. when used, when the substrate having the light receiving part is different from the substrate having the signal processing part, the metal bump connections are likely to break due to the difference in thermal expansion coefficients of those substrates.
Recently, a monolithic infrared imaging device, which solves the problem of the hybrid type device, has been proposed.
FIG. 7 is a cross-sectional view showing a monolithic infrared imaging device disclosed in Japanese Published Patent Application No. Sho. 63-46765. In FIG. 7, a first CdHgTe layer 12 having a small energy band gap and a second CdHgTe layer 13 having a large energy band gap are disposed on the CdHgTe substrate 11. The second CdHgTe layer 13 is partially removed to expose the first CdHgTe layer 12 on the surface, and a photodiode 14 is formed in the exposed layer 12 by ion implantation or the like. A signal charge injection layer 22 is formed in the second CdHgTe layer 13 by ion implantation or the like and a charge transfer gate 23, a charge storage gate 24 and a CCD 25 are disposed on the second CdHgTe layer 13 via the insulating film 26. The signal charge injection layer 22 and the photodiode 14 are connected by an indium electrode 21. When infrared light is incident on the photodiode 14 formed in the first CdHgTe layer 12 having a small energy band gap, signal charges generated in the photodiode 14 and flow through the indium electrode 21 and signal charge injection layer 22 the charge transfer gate 23, charge storage gate 24 and CCD 25.
Meanwhile, FIG. 8 is a cross-sectional view showing a structure of the monolithic infrared imaging device disclosed in Japanese Patent Published Application No. Hei. 2-272766. In FIG. 8, a second CdHgTe layer 13 having a large energy band gap is disposed on a semiconductor substrate 11. A first CdHgTe layer 12 having a small energy band gap is buried in the second CdHgTe layer 13. A photodiode 14 is formed in the first CdHgTe layer 12, and a source diode 18 and a drain diode 17 are formed in the second CdHgTe layer 13 by ion implantation. A connection electrode 15 connecting the photodiode 14 with the drain diode 17 and a gate electrode 16 connecting the source diode 18 with the drain diode 17 are disposed on the insulating film 19. When infrared light is incident on the photodiode 14 in the first CdHgTe layer 12 having a small energy band gap, signal charges are generated in the photodiode 14 and flow through the connection electrode 15 to reach a MIS (Metal Insulator Semiconductor) switch 20 disposed on the second CdHgTe layer 13. By opening a gate of the MIS switch including the drain diode 17, source diode 18 and gate electrode 16, signal charges are transferred to a signal output electrode (not shown) connected to the drain diode 17 and then output by a charge transfer device (not shown).
The monolithic infrared imaging devices shown in FIGS. 7 and 8 solve the above-described problems of the hybrid imaging devices. In addition, since the monolithic infrared imaging device detects light incident on the rear surface of the substrate, the semiconductor substrate is not restricted to transparent substrates, thereby increasing the degree of freedom in selecting materials.
In these monolithic infrared imaging devices, in order to improve the sensitivity to infrared light in the 10 micron wavelength band, the photodiode is disposed in the semiconductor layer having an energy band gap as small as 0.1 eV. Further, in order to suppress dark current (leakage current) in the MIS structure, the signal processing part including the CCD and the MIS switch are formed on the semiconductor layer having a large energy band gap.
In the monolithic infrared imaging device shown in FIG. 8, although the leakage current in the MIS switch is reduced, since the photodiode 14 is disposed in the semiconductor layer 12 having a small energy band gap, 0.1 eV, and both ends of the p-n junction constituting the photodiode 14 are present at the surface of the semiconductor layer 12 having the small energy band gap, a surface leakage current is induced. As a result, signal charges are not transferred to the signal processing part with high efficiency. Further, the photodiode 14 in the semiconductor layer 12 is connected to the MIS switch including the source diode 18, drain diode 17 and gate electrode 16 via the connection electrode 15 and an end of the p-n junction of the photodiode 14 and its vicinity are covered with the connection electrode 15, so that the numerical aperture of the photodiode 14 is decreased. In addition, since the connection electrode 15 and the MIS switch 20 occupy a large area of the light receiving surface, the light receiving region is reduced as the integration density is increased. As a result, light having a small area cannot be detected accurately.
In the monolithic infrared imaging device shown in FIG. 7, since the both ends of the p-n junction of the photodiode 14 are covered with the semiconductor layer 13 having a large energy band gap, the surface leakage current is suppressed. However, since the signal charge injection layer 22 in the semiconductor layer 13 is connected to the photodiode 14 in the semiconductor layer 12 by the columnar electrode 21, the surface of the photodiode 14 is partially covered with the electrode 21, whereby the numerical aperture of the photodiode 14 is decreased. In addition, since each end of the electrode 21 is present on a different plane, the connection precision of the electrode 21 is lowered, with the result that the reliability of the device is reduced.
Furthermore, in the devices shown in FIGS. 7 and 8, since the light receiving surface of the photodiode 14 is exposed on the surface of the semiconductor layer 12 having a small energy band gap, signal charges generated in the vicinity of the p-n junction are likely to reach the light receiving surface and recombine, so that the signal charges cannot be processed with high efficiency.